Charles Prescott (EA) is retiring on December
1, after 34 years
at SLAC leading the Lab and the world in developing and using
polarized sources of electrons in high energy physics experiments.
Prescott is probably most known for developing
the first high-intensity polarized source and heading the
groundbreaking E-122 experiment that solidified the then-disputable
electroweak theory, the foundation of today’s wildly successful
He also gains recognition each summer as one of
the four directors of the SLAC Summer Institute. He will complete
his tenth year as director next summer before handing the reins over
to someone new.
“SSI is hard work and it’s rewarding,” Prescott
said. “We get lots of positive feedback from lecturers and
In true SLAC fashion, the ‘retired’ Prescott
will still work half time (on ‘recall to active duty’).
Relinquishing the administrative duties of Group A Leader
(coinciding with the group’s dissolution), he will continue to do
R&D for the neutrino experiment EXO—a career anomaly because the
project uses no polarized source and no accelerator.
From Bubble Chambers to Polarized Beams
joined SLAC in 1971 as a research associate first working on the
BC-42 bubble chamber experiment in Group A. He soon turned to
polarization. In the 1970’s, experimental physicists wanted to probe
for quarks—not yet a solved mystery—using deep inelastic scattering.
In deep inelastic scattering,
off protons, interacting with the quarks inside, at energies strong
enough to break up the proton.
“Weinberg and Salam had a theory that unified
the weak and electromagnetic forces,” Prescott
said. “The theory predicted that in electromagnetic interactions,
you should see a bit of the weak force.” Prescott
proposed an experiment to test the theory of electroweak
interactions by finding parity nonconservation— violation of mirror
symmetry—in inelastic electron scattering.
To make the measurements, physicists needed a
source of polarized electrons. In a polarized particle beam, the
majority of the particles are aligned to spin in the same direction,
like a clockwise spiral on a football as it speeds toward a
receiver. Physicists expected that electrons polarized to be
forward-spinning would interact with quarks at a slightly different
rate than reverse-spinning electrons (a parity violating effect).
Because there was no way to polarize electrons
into an intense enough beam, Prescott,
then a permanent staff physicist, started work in 1974 with Roger
Miller (ARDA), Ed Garwin and Charlie Sinclair (both PEL) to develop
the first high-intensity polarized source.
They put their source to brilliant use in SLAC
experiment E-122 in 1978. It was the first high-intensity polarized
source on an accelerator and also the first use of gallium arsenide
(a semiconductor material) for an accelerator. When struck by
intense laser light, gallium arsenide emits a large number of
polarized electrons. Most electron accelerator labs now use the
material in their polarized sources.
The weak force has a different effect depending
on whether you look for it with a forward-spinning electron (that
you scatter off a
proton) or a reverse-spinning electron. The experiment famously
found this small asymmetry (10-4), thus proving that the weak force
is involved in electron-quark interactions.
E-122’s success provided the cornerstone for
acceptance of the electroweak theory, which at the time had
competitors. A year later, in 1979, Steven Weinberg, Abdus Salam and
Sheldon Glashow won the Nobel Prize for this theory. Prescott
won the 1988 Panofsky Prize for his experimental work.
Sinclair working on the
first high-intensity polarized source in the mid-1970's.
(Photo by Joe Faust)
Fixed Target and Colliding Beam Experiments
became an associate professor and contributed to proposals and R&D
for the PEP accelerator and a PEP detector. He was a charter member,
along with Marty Breidenbach, Dave Hitlin, Harvey Lynch and David
Leith, for SLD, the detector for SLC, the world’s first linear
proposed that the machine use polarized beams.
“The electromagnetic force is the largest force
when you scatter
off a proton, as in E-122, and the weak force is weak,” he said.
“But at the Zpole (where Z particles are produced) in SLC, the weak
force is by far the dominant force. The roles changed. Parity
violation is a very large effect. Its full nature stands out.”
The polarized source for SLD took full
advantage of advances in gallium arsenide made by Takashi Maruyama,
Garwin and others, which together with a state-of-the-art laser
enabled around 80 percent polarization of the electron beam.
The SLD collaboration published its complete
and final results this summer together with the final results from
similar experiments done at CERN in Switzerland. The polarized beam
enabled SLD to make key measurements more precisely than any other
experiment, even though SLD produced far fewer Z particles than the
“At the Z pole, we had the best tools, we had
the polarized electrons, we did the best measurement of the mixing
parameter called the weak mixing angle,” Prescott
said. The weak mixing angle is a ‘free’ parameter in the Standard
Model whose value is not specified; it gives important information
about the strength of the electroweak force and is a powerful tool
for predicting the mass of the still sought after
In parallel to SLD, Prescott
returned to End Station A, collaborating with Ray Arnold and his
group on learning the spin structure of protons and deuterons (a
hydrogen with one proton plus one neutron) using polarized electron
beams and polarized solid targets. They measured the spin of the
quarks in the proton and the deuteron.
In 1986 Prescott
became a full professor, and was associate director of the Research
Division from 1986 to 1991.
“He’s very good, and he’s somebody I always
trusted as a physicist and as an administrator,” said director
emeritus Burton Richter (DO).
Current Research Continues
For the past three years, Prescott
has been working on EXO to learn more about neutrino mass. Although
EXO does not use an accelerator, the experiment still fits well with
for probing the nuclei of atoms. EXO involves detecting certain rare
decays from the nuclei of xenon atoms.
“The physics is interesting and worth doing and
I like technically challenging R&D projects,” he said, neatly
summarizing his four decades in physics. Prescott now (shown above)
and in 1975.